Method and apparatus for dielectric heating



July 18, 1950 ca. E. GARD METHOD AND APPARATUS FOR DIELECTRIC HEATINGFiled Jan. 24, 1947 I N VEN TOR.

Patented July 18, 1950' UNITED STATES PATENT OFFlCE METHOD AND APPARATUSFOR DIELECTRIC HEATING Application January 24, 1947, Serial No. 724,186

This invention relates to dielectric heating and; in particular, to amethod and'apparatus for maintaining substantially uniform distributionof the lines of force through the mass to be heated.

In heating certain masses'bydielectric effect the distribution ofthelines of force through the mass is not uniform and the rate ofgenerating heat differs in various portions of the mass. 'This isundesirable in many cases-where uniform heating throughout the mass'isnecessary to obtain a uniform product. 'The-nonuniform distribution 'oflines 'of force is increased when the mass has a; dimension normal tothe usual electrode plates which isof the same order as or greater thanthe transverse dimension. In such cases, the lines of force through themass are spread farther apart-adjacent the median transverse plane thanat the surfaces of theelectrodes. As a -result,'more heat isgenerated'adjacent the electrodes than in the intermediate'portionof'the mass.

I have invented a novel method and apparatus for dielectric'heatingwhereby substantially uniform distribution-of the lines of force throughthe mass to be heated may be obtained even though thedimension of themass normal to the electrodes 'is several times the'other dimension. Ina preferred embodimentyl'provide a Voltage divider coil in the form of ahelix surrounding the mass to be heated, and connect the ends of thecoilto the electrodes, thus placing the inductance of the coil inparallelWith the capacity of the mass to be heated. Theinductance thustends to balance the load capacity. It also reduces the non-uniformityof distribution of the lines. of force through the mass which wouldexist if the mass were outside of the coil. By properly spacing thesuccessive turnsof the coil, a high degree of uniformity in thedistribution of lines of force may be obtained. A complete understandingof the invention may be obtained from the following detailed descriptionand explanation which refer to the accompanying drawing illustrating apreferred embodiment andcertain modifications. .Inthe drawings,

Figure 1 is a plan View;

Figure 2 is a section therethrough taken along the-plane of line II-IIof-Figurel;

Figure 3 is a view simi1artoFigure2show- -inga modification; and

Figure-4- is'a view of the -cross-section of-the embodiment in "Figure-1, illustrating closer 14 Claims. (01. 219-47) a 2 spacing of thesuccessive coil turns adjacent .the top and bottom portions of the mass.

Referring in detail to the drawings and, for the present, to Figures 1and 2, a mass IDof dielectric material to be heated is disposed betweenspaced electrode plates II and I2. An

insulating layer I3 is disposed between the mass and each electrodeplate. If the mass is composed of flowable material, e. g., corkparticles coated with a heat-activatable'binder, adapted to be convertedinto a solid massofcork composition, itis preferably confined in a moldl4 0f Which the electrode. plates II and I2 serve as the top and bottom,respectively. The mold may be of any suitable'shape or dimensions. Inthe form illustrated, it is.a cylinder composed of laminated paperimpregnated with ceresin wax so as to have a loss factor equal to orgreater than that of the material of whichthe mass In is composed. Amold having a loss factor equal to or greater than the material to bedielectrically heated is disclosed and claimed in my copendingapplication .Serial No. 678,217 filed June 21, 1946, and entitled Methodof Controlling Dielectric Heating (Prevention of Heat Loss). Dielectricheating equipment including a mold having a wax impregnant such asceresin is disclosed and. claimed in th copending application of GeorgeW. Scott, Jr., Serial No. 678,215, filed June 21, 1946, and entitledMethod and Apparatus for Dielectric Heating. In the curing of corkcomposition, the coated cork particles are compressed between theelectrode plates and the latter are held in position by through pins 15inserted through holes inthe mold wall.

A voltage divider in the form. of a helix l6 of any suitable conductoris disposed about themold i4 and'preferably in contact therewith or.partly or wholly embedded therein. The ends of the helix areelectrically connected to the electrode plates. In the illustratedembodiment, the end turns of the helix-are in contact with -the.pins 15which are of metal and are also in. contact with the electrode plates ll and 12. A source of high-frequency current I! suchas-a vacuumtubeoscillator is connected across the. electrode plates and the coil iii inparallel therewith by conductors attached to the pins [5. .The coil andplates may, of course, be separately connected in parallel to the sourceif desired. -If theinductance of the coil 16 is greater than needed tobalance the capacity of the mass lil,:in ordertoproduce a condition ofparallel resonance, avariable inductance 18 may be connectedinparallelwith the load and the coil It to reduce the overall inductancefor the purpose of tuning the circuit to resonance.

The coil H5 connected to the source of highfrequency H fordielectrically heating the mass constitutes a very much higher impedancethan the mass. At the relatively high frequencies employed in dielectricheating work, the stray capacitance between the mass Ill and the coil [6capacitatively couples all points of the coil [6 to the mass [0, andthere is thus provided a voltage divider which is capacitatively coupledto the mass at an infinite number of points.

The coil 16 performs two important functions.

In the first place, it serves to maintain substantially uniformdistribution of the electric lines of force through the mass In betweenthe electrode plates H and I2. This improves the uniformity of heatingthroughout all portions of the mass. This action results from the factthat the voltage divider coil, 16 confines within its turns thehighfrequency electric field of the condenser formed of theelectrodeplates H and I2 separated by the mass Into be dielectricallyheated. The second function results from the fact that, because of theimpedance constituted by various sections of the voltage divider It allconnectedin series, there isbrought about a uniform voltage gradienttherealong whenthe turns are uniformly spaced, and the voltage gradientin the coil determines the position of the equipotential surfacesthrough the mass. This follows from the fact that the voltage divider.establishes a high-frequency boundary potential distribution in thespace closely adjacent to and surrounding the mass; and Where the turnsof the voltage divider coil [6 are equally spaced, its impedance valuewill be uniform along its length, and it will possess a uniform voltagegradient which will establish a similargradient'within the mass, for thevoltage dividerv controls the electric field between the electrodesinaccordance with its, voltage gradient. This gradient may bevaried bychanging the spacing of the coil turns in various portions of its lengthto produce a substantially uniform temperature in the mass. Where theturns of the voltage divider coil I6 are spaced closer together, theimpedance value. will be higher, and a proportionately greatervoltagedrop will occur Thus, the voltage gradient will be nonuniform anda higher heatingrate will be obtained in the mass in the zones where thevoltage drop in the voltage divider coil 16 is greater because, asmentioned above, the voltage gradient of .the coil controls the electricfield within the mass. Thus, if it is desired to heat one portion of themass to a higher temperature than another, this result may beaccomplished by properly distributing the turns of the. coil,,therebychanging the voltage increased or the thickness of ,the mold walldecreased. The latter is particularly desirable to reduce the amount ofenergy consumed in heating the mold itself as contrasted with thecontents thereof.

The coil N5, of course produces a highaitaii frequency electromagneticfield longitudinally through the mold ID. This field tends to heat theelectrode plates H and I2 by induction. While some heating of theelectrode plates is desirable to prevent excessive loss of heat theretofrom the mass 10, overheating of the electrode plates is undesirablebecause of the danger of overcuring the top and bottom of the mass oreven charring it. To avoid this, I provide the electrode plates II andI2 which may conveniently be composed of steel, with peripheral rings orbands IQ of copper or other suitable material having a conductivity muchgreater than that of the metal of which the plates themselves arecomposed. The copper rings act in the known manner to shield theelectrode plates from the electromagnetic field. The alternatingelectromagnetic field thus induces current largely in the rings idrather than in the electrode plates themselves.

In using the apparatus described above for performing the method of myinvention, the mold M is filled by the aid of a so-called charging bootwith the electrode I I and upper insulating layer l3 removed. Theinsulating layer and electrode plate are then placed on top'of thecharge and it is compressed to the desired degree after which the upperpins I5 are inserted to hold the plate in place. High-frequency voltageis then applied as by closing a switchzil. When the mass has been heatedsufiiciently, the switch is opened, the plate H and upper insulatinglayer are removed and the mass is then pushed out of the mold.

Figure 3 illustrates a modification of the apparatus already described.The modified apparatus is adapted to be used between the'platens 2| and22 of a press instead of relyi'ngon through pins to hold and confine thematerial. As shown in Figure 3, a bottom electrode plate 23 is disposedon the platen 22. A top electrode plate 2t carried on the upper platen2| is adapted'to enter a mold 25 and compress a charge of material 26therein. Insulating layers are disposed "between the electrode platesand the material'to be heated. The mold 25 may be similar inconstruction to the mold l4 and is provided with a surrounding helix 2'!which serves as an inductance to balance the load capacity, having itsends connected to the electrode plates 23 and 2d, respectively. In otherrespects, the arrangement shown in Figure 3 is similar to that of Figure2.

Figure 4 illustrates the embodiment shown'in Figures 1 and 2 with,however, closer spacingoi the successive turns of the coil Hi alongthetop and bottom portions of the mass it! than the spacing of the turnsadjacent the intermediate portions of the mass. By such'spacing, aproportionately greater voltage drop with consequently greaterdielectric heating is achieved in the transverse portions of themass atthe top and bottom within the space surrounded by the closer spacedturns. In this way, a more uniform temperature of the mass Ill isobtained in areas of uneven and possibly greater heat loss as byconduction through the electrodes. Moreover, by

presetting such spacing as andwhere desired along the length of themass, variations from the uniform which may be wanted under specialcircumstances can also be achieved.

desired distributionof lines offorce or position of the equipotentialsurfaces through the mass. In the second place the voltage dividercontrols the electric field within the mass between the.

electrode plates in accordance with its voltage gradient and permitscontrolled-heating of the.

mass to obtaina substantially uniformtemperature in the mass or a highertemperature in one.

portion of the mass thanin another. In addition, the helix forming theinductance serves mechanically to reinforce the-mold wall, permitting are duction-in the sectionthereofwithout seriously weakening it.. Amoreuniform product is thus obtainable by the use of the invention withoutvfor supplying high-frequency alternating electric.

current tosai'd-w electrode. plates froma source to establish ahigh-frequency electric field within said mass between said electrodeplates, and meanslfor controlling the. voltage gradient within saidmass. between said electrode plates comprising a voltage divider coupledto said high-frequency current source and disposed closely ad.- jacentto and surroundin said mass between said electrode plates.

2. Apparatus for dielectrically heating a mass of dielectric materialcomprising a pair of substantially parallel electrode plates adapted tobe positioned on opposite sides of said mass, means for supplyinghigh-frequency alternating electric current to said electrode platesfrom a source to establish a high-frequency electric field within saidmass, and means for controlling the voltage gradient within said massbetween said plates comprising a voltage dividerv coil having aplurality of turns disposed closely adjacent to and surrounding. saidmass between said electrode plates and coupled to said high-frequencycurrent source.

3. Apparatus for dielectrically heating a mass of dielectric materialcomprising a pair of substantially parallel electrode plates adapted tobe positioned on opposite sides of said mass, means for supplyinghigh-frequency alternating electric current to said electrode platesfrom a source to establish a high-frequency electric field within saidmass, and means for limiting the distribution of the electric lines offorce of said field essentially to within said mass between saidelectrode plates and for controlling the voltage gradient within saidmass between said electrode plates comprising a voltage dividerproviding a plurality of closely spaced conducting surfaces disposedclosely adjacent to and surrounding said mass between said electrodeplates and coupled to said high-frequency current source.

4. Apparatus for dielectrically heating a mass of dielectric materialcomprising a pair of substantially parallel electrode plates adapted tobe positioned on opposite sides of said mass, means for supplyinghigh-frequency alternating electric current to said electrode platesfrom a source to establish a high-frequency electric field within 15said-mass, and means .for limiting the .distrlbue' tion -of thevelectric lines offence-of said field essentially to within said massbetween saidelectrode plates comprising a voltage divider. coil. havinga-plurality of closely spaced turns dis,-

posed closely adjacent. to. and surrounding. said mass-between. saidelectrode plates and con.-.

nected to said electrode platesin parallel therewith.

5-. Apparatusfor dielectrically heating a mass of dielectric materialcomprising a nonconductive container for said mass, electrode platesclosing said container and adapted to confine said mass within saidcontainer, means for supplying high-frequency alternating electriccurrentquency alternating electric current to said: elec-.

trode plates from a. source to establish a highfrequency electric fieldwithin said mass, and;

means for. limitingsaid field essentially to within said mass and forcontrolling the voltage gradient within said mass between said electrodeplates comprising a voltage divider coil. having aplu-- rality orclosely spaced turns between said electrode plates. which are disposedclosely adjacent to and. surround. said mold and are connected to.

saidelectrode plates in parallel therewith.

7...Apparatusfor dielectrically heating a mass of dielectricmaterialcomprising a pair of substantially parallel electrode plates adaptedtobe positioned on oppositesides of. said mass, means for supplyinghigh-frequency alternating electric current to said electrode platesfrom a source to establish a high-frequency electric field within said.mass, and means for controlling the voltage gradient within said massbetween said electrode plates comprising a voltage divider coupled tosaid high-frequency current source and disposed closely adjacent to andsurrounding said mass between said electrode plates, said voltagedivider having its conducting surfaces nonuniformly spaced to provide anonuniform voltage gradient in the voltage divider and in the electricfield.

8. Apparatus for dielectrically heating a mass of dielectric materialcomprising a pair of substantially parallel electrode plates adapted tobe positioned on opposite sides of said mass, means for supplyinghigh-frequency alternating electric current to said electrode platesfrom a source to establish a high-frequency electric field within saidmass, and means for controlling the voltage gradient within said massbetween said electrode plates comprising a voltage divider coil having aplurality of turns disposed closely adjacent to and surrounding saidmass between said electrode plates, and means connecting said voltagedivider coil to said electrode plates in parallel therewith to establisha tuned high-frequency resonant circuit with said voltage divider coiland the capacitance between said electrode plates.

9, Apparatus for dielectrically heating a mass of dielectric materialcomprising, a pair of substantiallysparallel electrode plates adapted tobe positioned on opposite sides of said mass, means for supplyinghigh-frequency alternating electric current to said electrode platesfrom a sourceto establish a high-frequency electricfield within said 1mass, means for controlling the voltage gradient within said massbetween said electrode plates comprising a voltage divider coupled tosaid high-frequency current source and disposed closely adjacent to andsurrounding said mass between said electrode plates, and a variableinductance connected in parallel with said electrode'plates for tuningthe high-frequency alternating electric current circuit to resonance.

10. Apparatus for dielectrically heating a mass of dielectric materialcomprising a pair of substantially parallel electrode plates adapted tobe positioned on opposite sides of said mass, means for supplyinghigh-frequency alternating electric current to said electrode platesfrom a source to establish a high-frequency electric field within saidmass, means for controlling the voltage gradient within said massbetween said electrodeplates comprising a voltage divider coilhaving..'a plurality of closely spaced turns between saidelectrodeplates which are disposed closely adjacent to and surrounding said massand connected to said electrode plates in parallel therewith, and ametal conductor of lower electrical impedance than said electrode platesdisposed adjacent to and surrounding each of said electrode plates andelectrically connected thereto.

11. In a process of dielectrically heating a mass of dielectric materialdisposed between spaced electrode plates, the steps of establishing ahighfrequency alternating electric field within said mass between saidelectrode plates to dielectrically heat the mass and controlling theelectric field within the mass between said spaced electrode plates byestablishing a high-frequency boundary potential distribution in thespace around said mass between said electrode plates closely adjacent tosaid mass.

12. In a process of dielectrically heating a mass of dielectric materialdisposed between spaced electrode plates, the steps of establishing ahighfrequency alternating electric field within said mass between saidelectrode plates to dielec-a trically heat themass and controlling theelectric field within the mass between said electrode plates byestablishing a high-frequency boundary potential distribution in thespace around said mass between said electrode plates closely adjacent tosaid mass having a voltagegradient therealong which is predetermined inaccordance with the voltage gradient desired within said mass betweensaid electrode plates.

13. In a process of dielectrically heating a mass of dielectric materialdisposed between spaced electrode plates, the steps of establishing ahighfrequency alternating electric field withinsaid mass between saidelectrode plates to dielectrically heat the mass and controlling theelectric field within the mass between said electrode plates byestablishing a nonuniform high-frequency boundary potential distributionin the space around said mass between said electrode plates closelyadjacent to said mass to effect predetermined voltage'drops within saidmass in accordance with the voltage drops in said boundary potentialdistribution.

14. In a process of dielectrically heating a mass of dielectric materialdisposed between spaced electrode plates, the steps of establishing ahighfrequency alternating electric field within said mass between saidelectrode plates to dielectrically heat the mass and confining andcontrolling the electric field within the mass between said electrodeplates by positioning a voltage divider electrically connected to saidelectrode plates closely adjacent to and surrounding said mass betweensaid electrode plates to establish an arbitrary high-frequency boundarypotential distribution closely adjacent to said mass.

- GEORGE E. GARD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,572,873. Allcutt Feb. 16, 19261,839,801 Northrup Jan. 5, 1932 1,839,802 Northrup Jan. 5, 19322,325,637 Stewart Aug. 3, 1943 2,426,053 Roberds Aug. 19, 1947

